This application relates to the treatment of wells in the production of oil, gas and other fluids from underground formations, and particularly to the stabilization of boreholes drilled for the production of hydrocarbons, and the stabilization of the fractured subterranean formations after fracturing. It includes the treatment of shale and clay in situ to prevent swelling caused by the absorption of water from fluids used in the drilling, completion, fracturing, stimulation and workover of wells.
Salts such as ammonium or potassium chloride are commonly used to make brines forced into wellbores to fracture hydrocarbon-containing subterranean formations. The fractures are propped open with various solid particulates, commonly carried into the fractures with the aid of viscosifying agents such as guar, guar derivatives, xanthans and other biopolymers or synthetic polymers. However, the inorganic salts sometimes are inimical to the presence of shale and clay in the formation, which may swell and slough on contact with the brine water.
A good description of the problem which this invention addresses in the context of formation drilling may be found in an article by Thomas W.
Beihoffer et al in the May 16, 1992 Oil and Gas Journal, page 47 et seq., entitled xe2x80x9cCationic Polymer Drilling Fluid Can Sometimes Replace Oil-based Mud.xe2x80x9d As stated therein, xe2x80x9c(S)hales can become unstable when they react with water in the drilling fluid. These reactive shales contain clays that have been dehydrated over geologic time by overburden pressure. When the formation is exposed, the clays osmotically imbibe water from the drilling fluid. This leads to swelling of the shale, induced stresses, loss of mechanical strength, and shale failure.xe2x80x9d Shale crumbling into the borehole (xe2x80x9csloughingxe2x80x9d) can ultimately place a burden on the drill bit which makes it impossible to retrieve. Fractures in the formation can become blocked by swollen and displaced clay and shale.
Salts such as potassium chloride have been widely used in drilling treatments to convert the formation material from the sodium form by ion exchange to, for example, the potassium form which is less vulnerable to swelling; also the use of high concentrations of potassium salts affects the osmotic balance and tends to inhibit the flow of water away from the high potassium salt concentrations into the shale. However, it is difficult to maintain the required high concentrations of potassium salts in the drilling fluids. In addition, the physical introduction of such salts causes difficulties with the preparation of the viscosifying materials typically used for drilling. Inorganic salts can also have a harmful effect on the environment if released.
Guar has also been used, as well as various other water-soluble polymers, in the treatment of wellbores and the earth formations surrounding them, particularly for the control of swelling and sloughing of clay and shale, and to transport proppant materials in fracturing and similar operations. In many applications, however, the guar tends to degrade in higher temperatures frequently found in the formations, namely temperatures of 150-375xc2x0 F. depending on the particular type of guar.
It would be desirable to assure that guar and guar derivatives will continue to perform acceptably at these higher temperatures.
The present invention includes the use of potassium formate together with guar for the treatment of clay and shale in subterranean formations during drilling and otherwise for the stabilization of clay and clay-containing shale. The combination of guar and potassium formate treatment extends the useful life of the guar in controlling swelling and sloughing of clay and shale in hydrocarbon recovery from the earth, and is particularly useful in formations having temperatures of 150-350xc2x0 F.